Optical cavities have many applications in optical telecommunications and sensing. The microcavities enabled by MEMS technology are able to achieve ultra-compact devices for different functionalities including filtering, refractometry, spectroscopy, biomedical optics and atomic studies. For most of these applications, the cavity has to be large enough for the insertion of the sample under test while still providing adequate quality factor over a wide spectral range. In this work we present an optical microcavity with improved optical performance using novel curved slotted micromirrors. The slotted mirrors enable an ultra-wide spectral range extending from the visible range to theinfrared range. The curved slotted mirrors, rather than the flat ones, improve the optical performance in terms of the side mode suppression ratio and3-dB spectral width. The microcavity is fabricated using deep reactive ion etching on silicon wafer with 140 µm etching depth optionally followed by metal sputtering on the mirrors sidewall to improve its reflectivity. The spacing between the two slotted mirrors is 140 µm enabling micro-fluidic tubes or fluid samples to be inserted in the cavity. The performance of the optical resonator is experimentally verified in the infrared region around 1550 nm wavelength. The curved mirrors improved the rejection ratio to about 20 dB compared to the about 3 dB for the flat mirrors, while the quality factor is close to 1000. The shift in the response of the resonator acting as a refractometer is measured due to the insertion of toluene in the micro tube. The presented structure opens the door for biological analysis on chip over wide spectral range.